1. Field of the Invention
The present invention relates to a zoom lens barrel, and more particularly to a linear guiding mechanism of a zoom lens barrel.
2. Description of the Related Art
A zoom lens barrel that is provided with at least one extending barrel which extends out forwardly while rotating to effect zooming is known. Such a zoom lens barrel is generally provided with a linear guiding mechanism for connecting two barrels, one of which is fitted onto the other, while guiding one of the two barrels relative to the other in an axial direction (i.e., in an optical axis direction) without allowing the one barrel to rotate relative to the other barrel. The linear guiding mechanism is generally comprised of a plurality of linear guide grooves and a corresponding plurality of guide projections which are formed on an inner periphery of the outer barrel and an outer periphery of the inner barrel, respectively. Each linear guide groove has a predetermined depth in a radial direction so as to receive a corresponding guide projection therein. In the case where the zoom lens barrel is provided with a focal length detecting device including a code sheet for detecting a focal length and a conductive brush which slidably contacts the code sheet, the code sheet is often provided to be fixed to the periphery of the barrel having the plurality of linear guide grooves.
FIG. 7 shows two barrels (an outer barrel 100 and an inner barrel 105) in a conventional zoom lens barrel which are connected to each other through a linear guiding mechanism comprised of a plurality of linear guide grooves 101 (there is only one of them seen in FIG. 7) and a corresponding plurality of guide projections 106 (there is only one of them seen in FIG. 7). The outer barrel 100 is provided with a circumferential recess 103 for receiving a code plate 102 therein. The code plate 102 is fixed to the base of the recess 103 therealong. As can be seen from FIG. 7, the circumferential recess 103 is formed on the outer barrel 100 to cross the linear guide groove 101 (which extends in a direction perpendicular to the circumferential recess 103) in a circumferential direction. In other words, the circumferential recess 103 and the linear guide groove 101 are formed on the outer periphery and inner periphery of the outer barrel 100, respectively, so that part of the circumferential recess 103 and part of the linear guide groove 101 are located at the same position in a circumferential direction.
Each linear guide groove 101 has substantially a rectangular cross section and therefore has a uniform depth, and a corresponding guide projection 106 formed on the outer periphery of the inner barrel 105 is fitted in the groove 101. With the linear guide grooves 101 and the corresponding guide projection 106, the outer and inner barrels 100 and 105 can move along the axial direction relative to each other without any relative rotation therebetween.
It is necessary for a portion of the outer barrel 100 between the circumferential recess 103 and each linear guide groove 101 to have an enough thickness so as to achieve sufficient strength thereof. If the portion of the inner barrel 100 between the circumferential recess 103 and each linear guide groove 101 is not thick enough, the portion cannot achieve sufficient strength, which may cause the portion to warp when a conductive brush (not shown in FIG. 7) slides on the code plate 102. However, in order to ensure a connection between each linear guide groove 101 and the corresponding guide projection 106, a sufficient amount of engagement therebetween must be secured, and the depth of each linear guide groove 101 must be more than a predetermined depth. Accordingly, in order to achieve sufficient strength at each portion of the inner barrel 100 between the circumferential recess 103 and the linear guide grooves 101, the inner barrel 100 needs to be formed so as to have a sufficient thickness thereat, which is disadvantageous in regard to obtaining a small and compact designed zoom lens. In the conventional zoom lens barrel shown in FIG. 7, it is possible for the circumferential groove 103 not to be formed, and the code sheet 102 can be directly fixed onto the outer periphery of the outer barrel 100. In this case, it is also difficult to achieve sufficient strength at each portion of the inner barrel 100 between the outer periphery thereof and the linear guide grooves 101 if the outer barrel 100 is formed thin.